1. Field of the Invention
The present invention relates to an electrically conductive paste containing a copper powder subjected to anti-oxidizing treatment.
2. Description of the Prior Art
A copper-based electrically conductive paste is conventionally prepared by dispersing a copper powder (as the conductor) and a glass frit into an organic vehicle. Such a copper-based electrically conductive paste is used to form external electrodes on chip-type laminated ceramic capacitors and to form circuit patterns on ceramic substrates. To form external electrodes and circuit patterns, the electrically conductive paste is applied to a substrate by coating or printing, and after drying, the paste is baked in a non-oxidizing atmosphere (such as nitrogen) or a slightly oxidizing atmosphere containing a small amount of oxygen (less than 1000 ppm).
What is required of an electrically conductive paste of this type is that the copper powder contained therein should be protected from oxidation because oxidized copper powder adversely effects the printing performance of the paste, increases the resistivity of the resulting electrodes and circuits, and renders the resulting electrodes and circuits poor in solderability. A common practice to prevent such troubles is to subject the copper powder to anti-oxidizing pretreatment. This treatment is accomplished typically by dipping copper powder in a solution containing boric acid with stirring and mixing, followed by filtration and heating at 50.degree.-260.degree. C. so that a film of boric acid is formed on the surface of the copper powder. The solvent of the boric acid-containing solution is an alcohol (typically methanol).
The conventional anti-oxidizing method mentioned above has a problem. Specifically, in the case where a solution containing boric acid is prepared using an alcohol which dissolves boric acid very well, a volatile ester is formed. This results in vaporization of boric acid during stirring, mixing, and heating.
Japanese Patent Laid-open Publication No. 5-195005 discloses a method to address this problem. According to this method, copper powder, boric acid, and a solvent are mixed together, and the solvent is then evaporated from the mixture by a drying process. The mixture contains 0.01 to 0.1 wt % of boric acid. (calculated in terms of boron) of the combined weight of the boric acid and copper powder to be treated. The mixture also contains a sufficient amount of either ketone or hydrocarbon as the solvent in order to dissolve the boric acid completely. However, the disclosed method is complicated and requiring a long process time, which results in a high production cost.